(1) Field of the Invention:
The present invention relates to backflow-preventing valves (back-flow ring type) to be attached to screw heads of screw type injection-molding machines for use in injection-molding thermoplastic resins or thermosetting resins. More particularly, the present invention relates to the structure of backflow-preventing valves having excellent wear resistance and corrosion resistance, in which a ceramic member and a metallic member are integrally joined together.
(2) Related Art Statement:
In the injection molding, when a molten material is injected through a nozzle, a part of the material backwardly flows along a screw grooves due to a reaction force of an injecting pressure exerted upon the material. The backflow-preventing valve serves to prevent such backflow. That is, when in use, the backflow-preventing valve (back-flow ring type) is assembled into a screw groove at a tip end of an injection-molding screw together with a valve seat, and functions as follows:
First, when the resin is metered as shown in FIG. 2(a), an end face of a backflow-preventing valve 11 is spaced from a valve seat 12. While a screw 13 retreats under rotation, the molten resin forwardly passes through the gap between the valve and the valve seat. Then, at the time of the injection molding as shown in FIG. 2(b), the screw 13 is advanced under rotation, and the backflow-preventing valve 11 is rearwardly pushed due to the reaction force of the molten resin. Consequently, the end face of the backflow-preventing valve 11 adheres closely to the valve seat 12 to prevent the backflow of the resin.
As a material constituting the backflow-preventing valve, various kinds of treated steels such as alloy steels, stainless steel, Hastelloy, Stellite coated steels or the like have conventionally been used.
Recently, since filler-containing resins or heat-resisting resins have frequently been used as materials to be injection-molded, wear-resistant materials or corrosion-resistant materials have come to be necessary for screw head-constituting members in the injection-molding machines.
Further, the inside of a nozzle of an injection-molding machine is subjected to high temperatures around 300.degree. C. and high pressures of not lower than 1,000 kg/cm.sup.2. Therefore, use of corrosion-resistant, heat-resistant materials having high strength has been demanded.
However, the conventional material constituting the backflow-preventing valves, has drawbacks in that the material cannot withstand wearing caused when the valve rotates while being pushed to the valve seat during the injection molding, or that they cannot withstand corrosion and wearing when the valves are pushed against a strongly corrosive resin at high temperatures and high pressure. Therefore, the conventional materials have problems in that the backflow-preventing valves need to frequently be exchanged, and that operation efficiency is poor.
Since a metallic material, which can be applied to all sorts of resins is unavailable, it is necessary that screws and backflow-preventing valves made of various materials are prepared to meet various kinds of resins to be injection molded, and every time the kind of resin is changed, the screw and the backflow-preventing valve are also exchanged. Thus, operation efficiency drop remarkably.
In order to solve the above problems, it may be considered that backflow-preventing valves are made of a ceramic material having better heat resistance and corrosion resistance as compared to the metallic materials. However, the ceramic material has poorer toughness than the metallic materials. Thus, if a backflow-preventing valve is made of such a ceramic material only, the valve may be broken and not practically used.